A common issue most people run into these days is understeer. Whether it is because they bought the wrong chassis or the wrong driven wheels, it's still an issue almost everyone runs into due to how manufactures set the cars up from the factory. Hell, even a newer Mustang understeers these days, but there are ways to go about fixing it. The sad fact is not many people know the proper way to go about fixing understeer.
We'll use a Mitsubishi Evolution as an example for understeer. The car from the factory has multi-link rear, as well as MacPherson front. The motor, transmission, and the transfer case sit at the front (engine bay) and the weight distribution of the car the car is 60% front and 40% rear. There are various methods people use to go about reducing understeer, one popular way is to change the rear sway bar to a larger sway bar. Now physics says that using a bigger rear sway bar prevents the rear of the car from flexing as much, meaning the front will still flex while the rear stays more rigid. Having the front roll/flex is a good thing for AWD/FWD cars, with a RWD car you would want the rear to roll more than the front. There are a few problems with adding a rear sway bar however, using a stabilizer bar to correct chassis fault reduces the rear suspension from being able to work as efficiently. It reduces the suspension independence in the rear. The method I used last year to correct understeer was to driver mod it. Using left foot braking to plant the front down, it allowed the car to be neutral going through corners. More importantly it also allowed me to "brake boost" around certain corners by left foot braking, building boost, and by the time I'm ready to go WOT there wouldn't be very much turbo lag coming out of the corner. It's a tricky concept and even more difficult implementation; I'll go into depth about that later.
The last and best method to go about fixing understeer is to change out the rear spring rate to a stiffer rear spring. The reason this is the best method is because you're still allowing the suspension to stay independent, while reducing rear body roll. Now, this is where you need to understand what type of suspension you have before you can estimate what rate springs you'll need. The Evolution is multi-link, which means you must factor in motion loss. The spring rates that I went with were 10kgs front, and 12kgs rear. I'll have a video up in the next couple weeks to help explain how left foot braking, proper suspension and alignment settings help aid how to be faster around a road course. I'd love to hear everyone's inputs on how they went about correcting understeer as well, so feel free to start a discussion about this.